Electromechanical pressure switches have a number of advantages over its electronic counterpart. They are intrinsically fail-safe with their robust mechanical construction, and they are immune to Electromagnetic Interference (EMI) without any active electronic circuitry. Electromechanical pressure switches are commonly used for critical safety interlocks such as the atmospheric pressure switch (i.e., ATM Switch) for semiconductor manufacturing equipment.
An ATM Switch is a safety interlock for the process chamber of semiconductor manufacturing equipment to ensure the process chamber is at a safe pressure to open to the environment. SEMI S2 Environmental, Health, and Safety Guideline for Semiconductor Manufacturing Equipment recommends electromechanical devices for safety interlocks, such as ATM Switches, due to their fail-safe operation.
Fig. 1 is a diagram of a semiconductor manufacturing equipment showing various vacuum sensing and control elements. An ATM Switch is installed at the loadlock since it is open to the environment for the wafers to be transported to and from the equipment. UHP grade 316L stainless steel per SEMI Specifications F19 and F20 is commonly used for the wetted surface of ATM Switches to maintain the process as clean as possible without contamination.
An ATM Switch is the safety interlock to prevent potentially hazardous residual gases escaping from the process chamber to the environment by keeping the pressure inside the chamber lower than the atmosphere. The activation point of an ATM Switch is set slightly below the atmospheric pressure so that the chamber cannot be opened if the ATM Switch is activated.
There are two different representations of pressures depending on the reference of the measurements: absolute pressure and gauge pressure. Absolute pressure is a pressure measured with reference to zero pressure (i.e., absolute vacuum), so the value remains unchanged regardless of the environmental changes. Gauge pressure is a pressure measured with reference to the atmospheric pressure of the location where the measurement is taken. The atmospheric pressure varies with the weather condition and elevation of the location, so the value of Gauge Pressure varies accordingly as well.
Commonly used units of pressure measurement are psi (pounds per square inch), inHg (inches of mercury), and bar. The SI unit is Pa (pascal), which is N/m2 (newton per square meter). There are other units such as atm (atmosphere), mmHg (millimeters of mercury), and torr. To distinguish absolute pressure and gauge pressure, a or g is added at the end of the unit such as psia or psig. Torr is for absolute pressure and most commonly used for vacuum measurement.
Below is the equation of atmospheric pressure as a function of elevation assuming the temperature remains constant.
This shows the atmospheric pressure decreases logarithmically when the elevation of the location increases.
Below is a graph to illustrate the change of atmospheric pressure with the elevation.
Since the atmospheric pressure changes with the elevation and weather conditions, the pressure reference of an ATM Switch must be gauge pressure. Let us take two of Intel’s fabs, Fab D1X and Fab 11X, as examples to see why it must be gauge pressure.
Fab D1X is located at Hillsboro, OR, and Fab 11X is located at Rio Rancho, NM. The elevation of Hillsboro is approximately 150 ft. above sea level and the elevation of Rio Rancho is approximately 5,500 ft. above sea level. The atmospheric pressure on an average day at Hillsboro is around 29.8 inHg (757 torr) while it is around 24.5 inHg (622 torr) at Rio Rancho.
Let us assume the activation point of an ATM Switch of a loadlock used in both fabs is set to be 30 torr below the atmospheric pressure with pressure increasing. This will ensure the pressure inside of the loadlock is always lower than the atmosphere so that any residual gas in the loadlock does not come out to the atmosphere when it is open(ed).
If the pressure reference of the ATM Switch is absolute pressure, the activation point is 730 Torr regardless of the location of the fab by taking 760 torr as the atmospheric pressure. If this ATM Switch is used at Fab D1X, where the atmospheric pressure is 756 Torr, the activation point is 26 torr lower than the atmospheric pressure. Even though the pressure delta is 4 torr less than intended, the ATM Switch still functions as the safety interlock to keep the pressure inside of the loadlock lower than the atmospheric pressure.
But if this same ATM Switch is used at Fab 11X, where the atmospheric pressure is 622 torr, the activation point is actually 108 torr higher than the atmospheric pressure. This ATM Switch cannot reach the activation point and never functions as the safety interlock.
If the pressure reference of the ATM Switch is gauge pressure, the activation point is 30 torr lower than the atmospheric pressure regardless of the location/elevation. The activation point will be 727 torr at Fab D1X and 592 torr at Fab 11X respectively, and the ATM Switch will always function as the safety interlock even if the atmospheric pressure is changed.
It is important to understand the difference between absolute pressure and gauge pressure to select the right type of pressure switch for your needs. The difference of these two measurements is substantial as shown on the above examples.
Wasco has been supplying numerous models of UHP pressure switches with both gauge and absolute pressure references to all the leading semiconductor equipment manufacturers for over 30 years. There are other factors to consider when selecting a pressure switch such as material of construction, dimensions, process temperature, connector types, etc. Contact Wasco’s dedicated team (sales@wascoinc.com) for technical support to select the right pressure switch for your application.
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